This invention relates to a molding resin composition comprising an unsaturated polyester, a dialkyl ester of alkenyl succinic acid and if necessary, another polymerizable monomer showing high toughness and good coating properties.
Molding resin compositions named BMC (bulk molding compound) and SMC (sheet molding compound) comprising an unsaturated polyester resin as a major component, and one or more termoplastic resins, curing agents, polymerization inhibitors, fillers, coloring agents, thickening agents, fibrous reinforcing materials, and the like are molded into articles having various forms by various molding processes such as compression molding, transfer molding, injection molding, etc. under heat and pressure. These molding materials have advantages in that (1) design freedom is great, (2) corrosion resistance is good, (3) they have electrical insulating properties, (4) weight saving is possible, (5) physical properties can be designed freely, and the like comparing with metal materials, so that they are recently used often as materials for car body from the viewpoint of saving energy.
But since these molding compositions have high molding shrinkage, there arise various defects such as bad surface smoothness, sink marks on the surface of a molded article having a complicated reinforcing structure such as rib and boss, and the like. In order to overcome these defects, there has been proposed a process of using a highly reactive unsaturated polyester resin in combination with a thermoplastic resin such as a poly (methyl methacrylate) series resin, a poly (vinyl acetate) series resin, a poly-.epsilon.-caprolactone series resin, a polyethylene series resin, a polystyrene series resin, a saturated polyester series resin, and the like. But according to such a process, although the defects of bad surface smoothness and sink marks are solved, there arises another defect of causing remarkable lowering in toughness. Further, since a large amount of polymerizable monomer having a boiling point near a baking temperature of coating is used in said process, the unreacted polymerizable monomer volatilizes at an interface between a molded article and a coating film at the time of baking the coating film, which generates blisters and remarkably lowers the appearance of the molded article.
Therefore, an unsaturated polyester resin composition having both low shrinkage and toughness together with good coating properties has long been desired.